Physiologically Essential Pulmonary Surfactants: A Brief Physicochemical Account

Abstract

Pulmonary surfactant (PS) is a complex mixture of lipids and proteins that forms a monomolecular film at the lung-air interface. It serves dual purposes: (i) lowers the surface tension; and (ii) participates in innate immune defense of the lung. Surface tension lowering property of PS is achieved by forming a surface film (monolayer) which is highly enriched in dipalmitoylphosphatidylcholine. Among the four surfactant proteins (SP), SP-A and SP-D are hydrophilic, SP-B and SP-C are hydrophobic and are also biocompatible in all the mammalians. This article gives a brief account of the history of research on PS, anatomy of lung, alveolar metabolism, composition and methodologies adopted for in vitro evaluation of the PS.The possible molecular mechanism of film formation (adsorption), and of film adaptation to surface changes (phase transitions) during the process of respiration have been described in detail. Major disorders of the surfactant system related with its clinical consequence, and the potentials of surfactant therapy in the treatment of some of these disorders have been discussed.A brief account of the evolutionary development of pulmonary surfactants has also been presented. Keywords BackgroundIn the 1950s and 1960s, a respiratory disease, misleadingly named hyaline membrane disease (HMD), was the world's most common cause of infant morbidity, especially among the preterm babies. The infant morbidity led to the need of search for the understanding of lung function.Scientists and clinicians, working together, discovered the existence and necessity of pulmonary surfactant (PS) and then attempted to figure out how to overcome its dysfunction. The year 1929 is considered as the starting date in the history of PS when Kurt von Neergaard's experiments revealed that the required pressure for filling the lungs with air was higher than filling it with liquid [Von Neergard, 1929]. The alveoli can achieves near zero surface tension at the lung-air interface following the principle of the Young and Laplace. In 1946, it was reported that lung tissue has a remarkably high content of the lipid, dipalmitoyllecithin (currently known as dipalmitoylphosphatidylcholine). Studies on the effects of nerve gases on lungs, significantly contributed to the understanding of the physiology of PS [Clements, 1957]. It was proposed that alveoli, made of lung fluid material, can achieve substantial stability through the quantity and quality of the surface-active materials available at the lung-air interface. Subsequently, with the help of modified surface balance, it could be demonstrated that surface tension can drop to low values upon compression of surface films from lung extracts [Klaus et al., 1961]. There exists similarity in surface activity of dipalmitoylphosphatidylcholine (DPPC) and the phospholipids isolated from freshly lavaged bovine lung. HMD, now known as respiratory distress syndrome (RDS), is due to the deficiency or dysfunction of PS [Avery and Mead, 1959]. Since 1959, scientists from various disciplines flocked to...